The present invention relates to a process for coating a ferrous metal surface with an electrical insulating coating and to ferrous metal articles, especially electrical steel articles, having an electrical insulating coating thereon. More particularly, the present invention relates to a process for providing a thin electrical insulating coating on a ferrous metal surface, which coating comprises phosphate and nitrate anions and is cured at a low temperature and to the product thereby provided.
Electrical insulating coatings are commonly employed for electrical steels which are generally silicon or low carbon steels and are used, for example, in electric motors, generators, and the like. Electrical insulating coatings are, of course, employed on exposed surfaces of parts made of electrical steel in order to eliminate or minimize the conductance of electricity between adjacent steel parts. The insulating coatings desirably have a hard, smooth, glassy finish, good moisture resistance, and good electrical resistance. Desirably, the coatings also strongly adhere to the steel surface and are characterized by minimal dusting and are compatible with other components. Also desirably, the coatings can handle high temperatures, i.e. maintain their mechanical integrity and electrical resistance under high temperature conditions. It would also be advantageous if the electrical resistance coating were applicable to the steel at a low coating weight and a fast low temperature cure time of approximately 30 seconds to facilitate high speed line coating. Of course, the exact specification or characteristics required of an electrical insulating coating are determined by the intended use of the insulated part by the manufacturer or other user of the part.
While it is necessary that the insulating coating meet the requirements of the manufacturer or user, the cost of the coating is also important. Generally speaking, the expense of a coating can be minimized by application of low coating weights and efficient coating process steps. For example, it is desirable to employ a process for applying the coating which facilitates high line speeds such as by employing a short drying step and which avoids high energy usage such as by employing low temperature drying conditions.
In accordance with the present invention, an electrical insulating coating which is capable of meeting manufacturer or user requirements is provided on a ferrous metal surface. The insulative coating of the present invention can generally be employed at economical, low coating weights and can be applied with high speed line coating and with use of conservative amounts of energy.